Van RV Proposed Solar System Check

Hello, everyone. I think I've read enough to design a system without being completely foolish, buy we'll see!

Background and Limitations

I have a large Sprinter van I'm converting to an RV. The roof has enough room for 700W of monocrystalline panels while leaving space for a large vent fan. I'm looking at these 175W panels. (Feel free to suggest better/cheaper panels with free shipping!) I've resigned myself to needing to tilt the panels to get enough power, so I'm using the "Optimal Year Round" tilt values from http://www.solarelectricityhandbook.com at a couple of different latitudes for planning. (And who knows, maybe I'll buy some linear actuators and tilt them to ideal angles every month remotely!)

Power Usage

Just in case these tables look like garbage, I'll attach PDF print-outs as well. (They're in the first reply.) I wish this forum would let us keep colors and borders in tables, but it actively removes them.

Appliance

Qty

V

I

W

Efficiency

Hours

Total Wh

Dometic Fridge 40 qt (90F/41F)

1

12

0.64

7.68

1

24

184

Xantrex 1000W Inverter phantom draw

1

12

0.35

4.2

1

12

50

LED light (single)

1

12

0.25

3

1

4

12

LED lights (additional)

3

12

0.25

9

1

2

54

Maxxair Vent fan (med)

1

12

1.2

14.4

1

4

58

Maxxair Vent fan (low)

1

12

0.6

7.2

1

16

115

Tablet charging

1

12

0.5

6

0.85

2

14

Phone charging

1

12

0.5

6

0.85

2

14

Desk Fan (high)

1

120

39

0.85

4

184

Desk Fan (low)

1

120

23

0.85

8

216

Raspberry Pi

1

120

2.5

0.85

4

12

Dell Monitor (25% Brightness)

1

120

11

0.85

4

52

These are the most accurate numbers I could find for the various DC devices, and I measured the AC devices with a Kill-A-Watt. I tried to keep the hours at the maximum realistic usage. If I can find a good DC desk fan, that'd be nice, but barring that, I'll need to use an AC one. I sleep with a fan on 100% of the time--even in Winter--for the noise and breeze. As you can see, it's the single highest energy usage. Low vent fan most of the day, with perhaps medium when cooking. I'm assuming an 85% inverter efficiency, which is the most conservative number listed by Xantrex and other brands. (However, the end-to-end solar efficiency factor of 0.52 supposedly already accounts for this, but the difference of double-counting it is not particularly large.)

Battery Sizing

Grand Total Wh

965.3

Days

2.0

DOD

0.50

Effective Wh

3861.1

Batt Amp Hours

321.8

Here, I take the total Wh and multiply times 2 for a day of reserve power, and targeting 50% DOD. At 12V, this yields a desired battery bank size of 322 Ah. While there are some 6V deep cycle batteries about that size, I doubt I can beat the price of 4x Trojan T-105's making it 450 Ah. In my previous post I mentioned I have a source for them at $110 each plus core. That's too much battery, but as long as I don't use it, I should be fine, right? However, I will do a price check on 2x Trojan J305P-AC.

Comments

Here's where things get a little bleak. I
used the solar irradiance charts to calculate an average year to
highlight the worst case scenarios. I think October is the latest
I'd want to stick around in Rochester NY, and NC is the furthest North
I'd want to be in November, so here is a full year of values, switching
locations in those months, and again in March:

Location

Month

Insolation

Wh

Days power

Greensboro, NC

Jan

3.52

1281.28

1.33

Greensboro, NC

Feb

3.89

1415.96

1.47

Rochester, NY

Mar

4.01

1459.64

1.51

Rochester, NY

Apr

4.84

1761.76

1.83

Rochester, NY

May

5.15

1874.6

1.94

Rochester, NY

Jun

5.41

1969.24

2.04

Rochester, NY

Jul

5.59

2034.76

2.11

Rochester, NY

Aug

5.41

1969.24

2.04

Rochester, NY

Sep

4.84

1761.76

1.83

Rochester, NY

Oct

3.82

1390.48

1.44

Greensboro, NC

Nov

3.86

1405.04

1.46

Greensboro, NC

Dec

3.44

1252.16

1.30

The insolation values are for South-facing, "Optimal Year Round"
tilt for the two latitudes. I calculated the watt hours by taking 700W *
0.52 * Insolation. Days of power is the watt hours divided by the grand
total power usage above (965.3 Wh). Like I said earlier, I've been told
that 0.52 factor already takes into account inverter efficiency. It
doesn't make a huge impact to double count it, though. If I change the
efficiency to 1, my power usage drops to 891 Wh, and the worst "Days
power" above (Dec) rises from 1.3 to 1.4. Battery bank size drops to
297. All in all not a huge change, so I might as well keep the more
conservative estimate.

You
can see that I often can't pay back two days of usage in one day. This
concerns me a little. If I have good day, bad day, good day, bad day...
my batteries might not fully charge. This is also ignoring device usage
while the batteries are charging. That scenario is not very realistic,
though. I'll much more likely face a few days in a row of rain and I'll
be stuck using the generator regardless.

Please let me know if my ratios of batteries to power usage to panels are OK or if I'm off. Thanks!

Solar Controller

Assuming all of the
above looks OK, what controller do you all recommend? I see the
MorningStar ones mentioned on here a lot. Specifically the TS-MPPT-XX
ones. I'm still confused on how to size the controller.

These have similar specs and are cheaper, both are pretty high efficiency, This one at 20%. I'm not familiar with either, just passing this along. The warranty on these is 10 years, Neither say anything about UL rating... Was posted on another site that these ship for $25 each so a good savings if a good panel. I would call ahead, Solar Blvd also sells blemished panels at times. I'd check that these were 1st quality;

I'd say it's likely they are made in the same factory. These fom solarblvd appear to come with just a junction box with MC-4 connectors. stating that 1000mm cables are optional. This would be just fine for me avoiding an extra connection.

I would go with a fanless controller for an RV, Morningstar makes a good product, Schneider would be another option. These are 12 volt nominal panels so you could use a PWM if you wanted.

I would check into O2cool fans, I saw one still listed as 12 volts on Amazon. Here is a 'sure' 12 volt version;

The standard 10" are now 9 volt, I have 2 running now in my home, on for air circulation in the back bedroom with air conditioning and 1 in the window in the un-air condition area of the house. Even on wall warts they draw 10Watts or less.

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol

Another low wattage option is an Opolar. I have an 8" version that runs off a USB port. I've been putting together small portable solar system in hopes of having a small system to run a tablet, fan, light, phone charging using a 10,000 or 20,000 USB power bank and a 20 watt solar panel that is 6 volts nominal.

It has a timer built in and 4 fan speeds and a cycle through option (that nobody likes...lol) Works pretty well, and I like that it shuts off, I'll use it when crashing in my Scion Xb on trips to see mom. Here are posted energy levels from the company.

Here is the posted amp draw, by the company in Amazon [email protected]One speed: 0.17A two speed: 0.35A three speed:0.6 A four speed: 0.8A

Puts out quite a good bit of air. I usually leave it on 2nd speed and set the timer for over night. I have just let it run and my 10,000(claimed) power bank shows about 60% charge left after sleeping (likely 7 hours). Lots of variables there!

Not cheap... Though I got mine when they had a 25 or 30% off one item sale.

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol

The fridge consumption seems a bit low too me as well. I think Dometic makes a variety of AC and DC compressor type (likely Danfoss based), and absorption (LPG) types. Assuming the one under consideration is compressor, I'd expect ~400wh/day or so.

I have a similar compressor, but much more heavily insulated box, which keeps consumption down to around the 180wh/day range listed.

If the unit is absorption type, the consumption may be just be for electric lights etc, with cooling provided by LPG. These can be run on electric alone, but they're really inefficient and will take way more than 180wh/day running electric only.

The numbers came straight from the manufacturer. The amperage I listed is for the fridge set at 41°F in a 90°F environment, maintaining already cold items. It's has a compressor, and it only runs as needed. It's 0.16 amps when idle, and 3 or so when actively cooling. So, it must average out to 0.64 amps. I can't find those numbers right now, but I'll keep looking.

Here's one anecdote from the Amazon review: "90+ degree weather and in a hot SUV parked in the sun and it never used more than 28 amp/hrs over 24 hrs." That's 82% more usage than what I listed. But another anecdote said it's 0.47 Ah/h (once cool) in his living room.

You have to read a lot of reviews to find ones that list the amp-hour usage, because most people just list the amps when running. It obviously won't have a 100% duty cycle, so that's not useful information except for deciding what size fuse to use.

I don't know where I got 0.64 from. I would have sworn that's what I found somewhere else. At any rate, that's an increase of 29 Wh per day.

Now, obviously, that amperage doesn't include opening and closing it, or adding new room temperature food, but it's a good baseline. There are already so many conservative estimates in these numbers, I feel OK leaving this one optimistic. Especially if I take chilled food from a store and put it in the fridge immediately. And since it's top loading, you don't even lose the cold air when you open it. I also plan on buying the official insulated cover. I'm hoping that the quoted numbers don't already include using that.

Fans

If a USB fan can move enough air, that would be a large savings in power. I can always use my phone to generate white noise if the fan's not loud enough. (For my purposes, all these "Very quiet!" fans are not a good thing! )

Removing the desk fan from my math and adding the Oploar 8" on high for 12 hours brought my total energy usage down to 709 Wh/day. Almost 27% less! I really hope a fan like that works out. For only $30 it's worth trying out. It would raise my worst sun example (Dec in Greensboro) from 1.3 days of power up to 1.77 days.

For being almost the exact same size, and looking identical, I find it odd that the Newpowa ones are 16.3 pounds and these ones are 28.2 pounds. If they are good and $135 ea, shipped, then that's a good deal. Seems too good to be true, though. Looking at the page they are listed on, there are two listings for the same panel. One says they're in stock, and another says they're not. And the 160W ones are more expensive. It doesn't add up.

Controller

I'm still looking for specific advice on what controller size I need, not just brand advice.

These are 12 volt nominal panels so you could use a PWM if you wanted.

In that case, I would need to run them in parallel, right? There still might be some difference between the 14.8V bulk charging value for the batteries and the voltage these panels produce. Their Vmp is 19V, and I've read that maybe I'd get 17V out of that. The difference between that and 14.8V is lost with PWM, as I understand. My question is: is that taken into account at all with the 0.52 end-to-end loss factor? I don't want to be too conservative. Or does that already assume you're using MPPT?

How many amps does the controller need to be rated for to be safe and effective with the four panels I linked in the original post?

Overall

Ignoring specific usage, does my math seem OK? Would you be comfortable having only 700 W of panels given this usage and these batteries? (With or without the AC desk fan.)

The math looks modest, but man.... the stories I read on reddit, Amazon reviews, or YouTube videos, has people powering a fridge, two laptops, phones, a Maxxair fan, and whatever else, and they're like, "We have 300W of panels and a 150 Ah battery." -_- It makes me feel like I'm really over-engineering things, but I obviously don't know what their real usage is.

I would hate to not buy enough and find out I'm destroying my batteries while having no room for more panels and having to make difficult cuts in usage or run my generator all the time, but I'd hate it just as much if I spend $2500 and I find out I'm only using 10% of my batteries per day when I could have just spent less money.

I'm just looking for some reassurance that I'm at least in the ballpark, and I'm not missing something obvious. Especially regarding current and charge rate, etc. Things that aren't necessarily obvious when looking at raw numbers.

Of course you can run a fridge, blah, blah on 300w of panels and a 150ah battery. It works until it doesn't.

FWIW, I have 2x6v GC ([email protected]) batteries on my sailboat (no solar). With a Danfoss compressor based top load fridge, and normal lights, laptop, etc. (but no fan), that will get to ~50% SOC in 1-2 days. With 700w of solar, I could just about get to fully charged from 50% on a sunny day with well aimed panels and little/no shading. You don't need to get to fully charged every day. If you get there every week or so, and avoid leaving the batteries at a low SOC for long, that's ok.

If I was doing it, I'd probably start with a single string of GC batteries and see how it goes. If a 2 string configuration makes sense based on actual experience, you can easily add a second string in the first month or two, or if something really goes sideways, you've only lost a single string.

Your math looks about right to me. You may have to run the genny occasionally, but we (mostly) all do.

These are 12 volt nominal panels so you could use a PWM if you wanted.

In
that case, I would need to run them in parallel, right? There still
might be some difference between the 14.8V bulk charging value for the
batteries and the voltage these panels produce. Their Vmp is 19V, and
I've read that maybe I'd get 17V out of that. The difference between
that and 14.8V is lost with PWM, as I understand.

My question is: is
that taken into account at all with the 0.52 end-to-end loss factor? I
don't want to be too conservative. Or does that already assume you're
using MPPT?

How many amps does the controller need to be rated
for to be safe and effective with the four panels I linked in the
original post?

0.81 for warm to hot solar panels (freezing or subfreezing panel temperatures would be approaching 1.0 for MPPT controllers)... Also includes a few percent losses for dirty panels

0.95 for MPPT efficiency (0.81*0.95=0.77 happens to work out for PWM system efficiency for other reasons)

0.80 for Flooded Cell Lead Acid battery (use 0.90 for AGM or higher)

0.85 for "average" loading AC inverter efficiency (note that very light loading can have much lower efficiency/Tare losses)

Multiply all the numbers together:

0.81 * 0.95 * 0.80 * 0.85 = 0.52

The above numbers are for well designed systems running "nominal" configurations. If you do something "different", the numbers can change.

Note that panel Wattage is "Marketing Specifications" not NOTC or PTC derated solar panel ratings (NOTC and PTC approach the ~0.81% derating factor--They are a bit less conservative, as I recall). Some folks like to use 75% panel+controller derating (instead of 0.77)--A bit more conservative.

If you run your system outside of "nominal" operation specifications, your deratings will be different (run FLA at 90-100% SoC, efficiency falls; run 3,000 Watt inverter with 10 Watt load, effiency will be way down, etc.).

They are relatively conservative (i.e., most people will see "at least" the predicted performance). If you were designing a system that needed 100% of predicted loads (say a refrigeration system) and no variable loading (optional loads, varying loads, etc.), then the base power needs should be increased by 1/0.75 to 1/0.65 (1.33 to 1.54x larger solar array suggested)-- You do not want to use 100% of predicted harvest every day. Some days will be better, some will be worse.

As always, there are details... Check the manuals, and some vendors have a website that helps to configure the array to controller (maximum Voc-array voltage on cold days, Maximum Imp/Isc array input current, etc.).

So, that includes the 85% inverter efficiency. I've been including that efficiency factor in my power usage numbers since only half of my devices are AC. Since I'm already modifying my power usage to account for that, it doesn't sound like I should count it again, so I should probably be using something like 0.62 as my end-to-end efficiency factor, which is the product of the other three numbers.

note that very light loading can have much lower efficiency/Tare losses

By tare losses, do you mean the idle draw that's always there? I'm already including that in my power usage. Is that the only thing leading to a lower efficiency for light loads, or is there even more? I can imagine that when the inverter notices a 500 mW load from a phone charger, it starts drawing power from the batteries, and there being a minimum amount it can draw.

Fuses vs Breakers

If I go with the 4x Trojan T-105 in parallel, I was looking to get two in-line fuse holders and some 50A (or maybe larger) fuses to go at the ends of each string in case one string goes bad or gets disconnected. That way the other string will blow a fuse instead of taking the whole load. Then I thought about circuit breakers instead. It turns out that in-line 50A circuit breakers are about $13 on Amazon. That's as cheap as some ANL fuse holders and a couple of fuses. But are they trustworthy? That particular model I linked is re-branded by at least five different companies.

Do any of you use DC circuit breakers instead of fuses? Are there reputable brands you'd recommend? How much should I be looking to spend?

Those boxes and breakers look nice, but I think they're overkill for my little Van RV. I'll probably just stick with fuses or maybe a reputable brand of breaker. The ones from Blue Sea Systems look pretty good, and they double as a shutoff switch.

Wires and Voltage Drops

The panels I'm looking at have 3' #12 wire leads. With four panels that's 24' of wire all looped around on my roof. I'll need to add probably 18' more to get it down to the battery bank because of the width of the array. I can buy 15 feet of extension cables in #8 or #10. There's a $15 difference. Totally negligible considering the cost of the system, but money is money! Is saving 0.18% of a voltage drop over 18 feet worth $15? That's 0.126V out of 70V. At 9.65A that's 1.22W! I can't lose that!

.... No, I'm just going to buy the #10. Or, heck, I could save $5 more and buy the #12 and still have only a 2% drop over all 42'.

This is all assuming that I should be using the panel short circuit amperage in this math. Should I use more to account for the "unusual solar circumstances" I've read about? Or do you only worry about that for safety and controller size, etc? I've read you should add 25% to the amperage for that calculation. If I use the 12.06A in the math, my voltage drop jumps to 2.52%. That's a 21W loss in total.

I don't even know why I posted all of this. We're talking a few dollars here for a fatter wire. But please tell me if I'm way off and I should care more about fractions of a percent of a drop. I think because I'm planning on running the panels in series that it matters so little. With lower voltages wire size would matter so much more.

MorningStar Calculator

It turns out their array size calculator will happily let you enter a positive Voc temperature coefficient! I only noticed it because the resultant "Minimum Vmp (average high temp.)" was higher than the "Maximum Vmp (record low temp.)" Thanks, MorningStar!

I get that it's operator error, but it should validate the sign of the number. Maybe I'll drop them a line.

With the correct coefficient, my array is no longer "optimal" for the MS-MPPT-45. However, it's not a big deal. It only becomes suboptimal at -9°F or lower where the voltage rises to where controller can no longer harvest the maximum power point. It says it won't harm the controller. That's not something I'm too worried about.

With the controller I'm looking at, and the four panels, the only two choices I have are:

One string of 4

Two strings of 2

I can't wire them all in parallel. At least not according to the MorningStar calculator.

I had planned on wiring them all in series because a higher voltage results in a lower percent voltage drop. I hadn't considered the shading issue. I was already going to make sure there was no shading from vent fans, roof rack, etc. When I'm in a park or national forest, I'm likely screwed either way, unless I can find full sun.

Thanks for the tip! I supposed I'll plan my wiring assuming I'm going to use 2x2. With a couple of MC4 Y junctions handy I can always switch between the two configurations.

> With a couple of MC4 Y junctions handy I can always switch between the two configurations.

Just be sure the current is off, so you are not disconnecting the MC4's under a load. They will arc internally and burn the gold plate contacts. I'd forgot you were using the MPPT, so you do need to have enough voltage for the BUCK circuit to work.

I had planned on wiring them all in series because a higher voltage results in a lower percent voltage drop. I hadn't considered the shading issue. I was already going to make sure there was no shading from vent fans, roof rack, etc. When I'm in a park or national forest, I'm likely screwed either way, unless I can find full sun.

One drawback to having solar on your Camper is the need for full sun exposure vs. A hot interior space. Makes afternoon naps a bit hot at times.

One drawback to having solar on your Camper is the need for full sun exposure

I don't really have a choice! I don't see myself carting out 700W of panels every time I park the van.

As for the heat, I'll just wrap the van in space blankets! Joking aside, I'm going to have plenty of insulation inside. Hopefully it will be enough.

Just be sure the current is off, so you are not disconnecting the MC4's under a load.

Thanks! That's good to know. I was planning on having a shutoff switch on the solar power.

I just redid some of the math for a 2x2 array. When dropping the voltage to two 35V circuits, each 12' circuit of #12 wire that comes on the panels will have a 2.63% voltage drop. Then, even with #8 wire, the added ~18' of wire will have a drop of 0.78% adding up to a total drop of 3.41%. That's no ideal.

I suppose I'll just run them in series unless I find I have a lot of problems with partial shading.

MorningStar TS-MPPT-45

Here's another random question. It looks like this charge controller doesn't utilize a shunt for measuring the current going into the battery, but it has all sorts of metering features available if you buy the TriStar meters, which I had planned on doing. How does it accomplish the metering accurately? Also, what happens if you run devices at the same time you are charging? Does that throw off the meter's estimation of your SoC?